MRI apparatuses are diagnostic imaging apparatuses for medical use, which induce magnetic resonance of atomic nuclei in an arbitrary cross section of a test subject to generate magnetic resonance signals, and obtain a tomographic image from the signals. The apparatuses transmit a radio frequency wave (henceforth also referred to as high frequency wave or RF), a kind of electromagnetic waves, to the test subject to excite spins of the atomic nuclei in the test subject, then receive the magnetic resonance signals generated by the nuclear spins, and reconstruct an image of the test subject. The transmission is performed with an RF transmission coil, and the reception is performed with an RF reception coil.
In recent years, in order to improve SNR (signal to noise ratio) of the image, it tends to use a static magnetic field of higher intensity, and use of high magnetic field MRI apparatuses using a static magnetic field strength of 3 T (tesla) or higher (3T MRI apparatuses) begins to spread. However, as the static magnetic field intensity becomes higher, obtained images more easily suffer from non-uniformity of the images. This is because the frequency of RF used in order to induce the magnetic resonance phenomenon becomes higher with use of the higher magnetic field intensity. For example, the 3T MRI apparatuses use RF having a frequency of 128 MHz, and this wavelength of RF in living bodies is about 30 cm, which is in substantially the same scale as that of a section of the abdominal part, and gives phase change of RF in the living bodies. Therefore, irradiated RF distribution and spatial distribution of rotating magnetic field (henceforth referred to as B1) generated by RF to induce the magnetic resonance phenomenon become uneven to cause the image non-uniformity. Under such a current situation, there is desired a technique for reducing the non-uniformity of the distribution of the rotating magnetic field B1 at the time of RF irradiation performed in high magnetic field MRI apparatuses, in order to improve image quality.
As methods for reducing the non-uniformity of B1 distribution, several techniques of devising the RF irradiation method have been proposed. Among them, a technique called “RF shimming” has appeared and attracts attention in recent years. This is a method of using an RF transmission coil having two or more channels and controlling phases and amplitudes of RF to be applied to the channels to reduce the B1 non-uniformity in imaging regions.
In the RF shimming, in general, the B1 distribution of each channel is measured before the image acquisition, and amplitude and phase of RF for reducing the B1 non-uniformity are calculated by using that B1 distribution (refer to, for example, Patent document 1 and Non-patent document 1). There is also a technique of performing imaging by setting a region of interest (ROI) and using at least one of amplitude and phase of RF as an imaging condition so as to reduce the B1 non-uniformity in ROI (refer to, for example, Patent document 2). In the technique described in Patent document 2, when a plurality of ROIs are set, obtained is at least one of such amplitude and phase of RF that variation of data among the plurality of ROIs is reduced. A problem that the influence of the B1 non-uniformity differs depending on the characteristics of individual subjects is thereby solved.